Activated magnesia and method of making



Oct 29, 1940- M. Y. sEAToN 2.219326 ACTIVATED MAGNESIA AND vMETHOD 0FMAKING Filed Jan. e, 1940 2 sheets-smet 1 am K). l

Oct. 29. 1940. M, Y, SEATONn 2,219,726

' AcTIvA'rx-:D MAGNESIA AND METHOD oF MAXIM Filed Jan. 6, 1940 2sheets-sneer 2 /Od/'ne Number 012545576910 15 zo z5,v ao

TIME -MINU'IES POWDERED ACT/VE M90 77 ACTIVE PRODUCT,

` To PRCKAGING '3. v 3M K /zfax Yeayi 'Ona Patented Oct. 29, 1940 UNITEDsrrfrlas AICTIVTED MAGNES-IA AND METHOD 0F MAKING Max Y. Seaton,

Greenwich, Conn., assigner to Weatvaco Chlorine Products Corporation,New York, Ni Y., a corporation ol.' Delaware Application January s,1940. sei-m No. 312,179

14 Claims.

from'hydrated magnesia Aby heat in a current of 1 hot gases until theadsorbent power reaches 'a out change in volume.

desired value as shown by adsorption of iodine from carbon tetrachloridesolution, and wherein the'magnesia is then cooled and packaged; all asmore fully hereinafter set forth and as claimed.

vIn certain prior applications whereof the present applicationfis acontinuation-impart, I have described and claimed the manufacture ofactivated magnesia as anew adsorbent material having an'activitycomparable with, or exceeding, that of commercial decolorizing carbons;said material being made by the careful removal of VHiO iromhydratedmagnesiaA (magnesium hy. droxide). TheseA prior applications are asfollows,

the later being continuations-in-.part of the earlier: Seriall No.703,935, filed Dec. 26, 1933; Serial No. 26,005, led June 11, 1935;Serial No. 26,006, filed June 11,' 1935; Serial No.- 81,654, illed May.25, 1936; and Serial No, 104,231, filed Oct. 6.

Both hydrated magnesia and anhydrous magnesia in their usual forms havesomeadsorbent power but not' sufiicient for commercial purposes incompetition with good adsorbents.' I have, however, discovered that bymethodically dehydrating magnesium hydrate byV baking in a current'ofair or inert gases under controlled conditions I can produce a new typeof adsorbent material; what may be called activated magnesia.

Chemically, it is often vlargely MgO but the orig--v inal particle orgrain of hydrated magnesia Mg OH 2 persists physically, substantiallywith- Activated magnesia has a greater adsorbent power than boneblackand the earthy adsorbents largely used in the mineral oil industry;alumina `(bauxite), fullers earth, bentonite.v etc., etc. Its adsorbentpowers for many coloring matters are comparable with, and can be madeconsiderably greater than, those of the standard commercial decolorizingcai-bons, over which it also has the further advantage of being white'.

Activated magnesia is useful in decolorizing and purifying manynon-aqueousliquids. In liquids containing water it tends torevert toordinary hydrated magnesia.

v(Cl. 252-2) Magnesium hydrate or magnesium hydroxide, MgiOHn, retainsits water oi combination and does not dissociate be'low about 350 C. Thevapor vtension of H2O becomes significant. at that temperature andabove. In baking hydrated mag- 5 nesia, the loss of water depends upontime, temperature and the H2O partialpressure in the atmosphere incontact with the magnesia. In heating there a're two actions which cantake place, one being the loss of water and the other being a shrinkingaction. Dehydration is antecedent to shrinkage and theshrinking actionis not very rapid at temperatures below 450 C. In

a general way the greater theamount of water removed, the greater is theadsorptive power of 15, the product, shrinkage being equal.

Magnesium hydrate about 31 per cent-of combined H2O, or about 45 poundsH2O for every 100 pounds MgO. With complete dehydration .145 pounds ofMglOHlz becomelOO pounds of MgO. 2o In the present invention the bestproducts -are vmade by carrying'the dehydration not quite so far; byleaving a little residual H2O as a'sort 'of factor of safety againstshrinkage. Theproduct advantageously containsan amountof II-Izcorf sresponding to at least about 2 per cent residuall Mg(OH)2; 100 pounds oisuch product may be considered, for analytical purposes, as containing 2pounds Mg(OI-I)2 and 98 pounds MgO, plusv traces of impuritiesifpresent.y This corresponds to about 0.6 pounds'-H2O in 100poundssoffthe'- product as it leaves the kiln.

Often I vadjust conditions so that the product- .contains substantiallymore H2O. ,P roducts'containing H2O in amount 'corresponding to 10 -per$5 cent MgiOH) z and 90 'per cent M gO are especially good. Thisproportionam'ounts to about 3 pounds H2O in 100 pounds'of'the materialleaving the kiln. Activated magnesias in which 80 to 85 per cent of theoriginal Mg(OH)2 have been con- 40 verted to MgO are also good.In-making a product in which 80 per cent of the original MgtOH 2 hasbeen converted to MgO, 145 pounds Mg(OH) 2 yield on dehydration 109pounds of aproductv which may be regarded for analytical purposes ascomposed of pounds MgO and 29 pounds Mg(OH) z. This product containsabout 9 pounds of water, or about 8 pounds water for each 100 pounds ofthe product. I have obtained excellent activated magnesias which containas much as 80 50 per cent residual Mg(OH)2; corresponding to` about 25pounds H2O in 100 pounds of the product. It is not knownA in just whatstate the residual H2O exists in the activated product. Perhaps the 55or lessl micro-cellular; there are, so to speak,

molecular holes or porosities where the H2O I nolecule is removed.`These are probably responsible for adsorption.

For most commercial purposes high adsorbent power is required. Theadsorbent power can b'e adjudged by treatment of colored oils, but thereis so much observational error that I IlndI it better to base the teston adsorption of `iodine from a carbon tetrachloride solution, using asolution.of iodine in pure carbon tetrachloride, advantageously of about0.5 N concentration.

Enough of the iodine solution is provided, relative'to the quantity ofmaterial being tested, so

' that abstraction of iodine from the solution makes only a negligiblechange in the concentration. A specimen of the material is shaken withthe iodine solution understandardized agitation conditions for a i'lxedlength of time. up by a gram 0I material under these conditions can bedetermined by titrating the solution; and from this the iodine value'canbe calculated. I generally use as the iodine value the millimolecules(0.127 grams) of iodine taken up by a gram of material, multiplied by100. As a matter of coincidence, one very good grade of commercialdecolorizing carbon has an iodine value of 100; that is, one gram ofsuch material will take up about 0.127 grams of iodine. Iodine values ofor greater meet most commercial requirements. Ordinary dehydratedbauxite, fullers earth, etc., etc., have values far below 60 on thisscale. The best active A1203 has a value of 17, and the best activemagnesia made by direct calcination of magnesite has an iodine value ofonly 55. By the present invention, activated magnesias can be made on acommercial scale with iodine values oi' 145 or better.

There are, as stated, many variables in the dehydration of hydratedmagnesia. One important variable is the concentration of HzOin theatmosphere with which the magnesia is in contact during the baking ordehydration. Other important variables are time and temperature, whichhave a determinable (and generally reciprocal) relationship. The effectsof these variables, and the operation of the invention, will be furtherdiscussed with reference to the accompanying drawings, in which:

Fig. 1 represents graphically the relations be.

tween time, temperature and iodine number in dehydrating magnesia, theatmosphere being considered constant; with comparative data forcalcining magnesite;

Fig. 2 reproduces a portion of the graph of Fig. 1 to an enlarged scale;and

Fig. 3 shows diagrammatically one type of apparatus suitable for use inpracticing the present invention.

In the graph of Fig. 1, the abscissae represent time and the ordinatesiodine number. Curves in solid lines show the iodine number of samplesof magnesium hydroxide heated for various periods in a mufile furnace atfive different temperatures. The atmosphere is presumed to be constantthroughout these tests. 'I'he higher dehydration temperatures make forproducts of higher iodine numbers; but in employing the highertemperatures the heating time is critical. At

The amount taken I 781 C. for instance, the highest iodine number isobtained when the hydrated magnesia is given a ilash baking of 21/2minutes. When lower temperatures are employed, the heating time is not"so critical.

Curves for magnesite, heated ina munie furnace under comparableconditions, are indicated in dotted lines in Fig. 1 for the sake ofcomparison. Itis readily observed from these curves that the iodinevalues (adsorbent capacities) of magnesias prepared in this manner aregreatly inferior to those of the activated products of this invention.

Fig. 2 shows the left-hand portion of several curves from the graph ofFig. 1, but with an expanded abscissae scale, and shows more clearly thecharacteristics of the curves. It will be observed that the curves haverather pronounced maxima. This corresponds to the observed fact that ingeneral a product of a given iodine number (below the maximum) can beobtained by employing either of two different baking times. For example,with a heating temperature of 659 C., an iodine number of 100 isobtained with a heat-treating time of 11.5 minutes or one of 25 minutes.However, the maximum iodine value of 144 is only obtained by heating at659 C. for 16.5 minutes, followed by rapid cooling. It appears. that,after the maximum adsorbent power is attained, shrinkage resulting fromcontinued exposure to high temperatures tends to reduce it.

I'he curves of Figs. 1 and 2 indicate a fairly definite relationshipbetween the time and temperature required to produce products of maximumactivity, in` accordance with this invention. 'I'his relationship may beexpressed mathematically as follows:

Log time (minutes) :3.7 (0.00271 X K.)

wherein K. is the temperature in degrees Kelvin (=C. absolute). It isassumed of course that a temperature suiliciently high to produce therequisite dehydration is employed; such temperature being at least 350C. and usually above 400 C. The stated relation was determined bynumerous tests in a muille furnace, but is generally applicable if theconstants are replaced by other values dependent on the particularequipment employed for the dehydration. In kiln operations the relationsof time, temperature, atmosphere, etc., are empirical; but theabovestated relation is applicable on a qualitative basis, at least. A

Kiln operations are the most convenient method of producing my improvedactivated magnesla on a commercial scale, and I have found that they canbe conducted in such a manner that material of any given quality (iodinevalue) can be reproducibly made. In such operations, hydrated magnesiain suitable form is fed into the upper end of a rotary inclined kiln andpassed downward through the kiln against a counterflow of hot flamegases. The hydrated magnesia may be in the form of a press-cake orslurry, and may be made up of freshly precipitated hydroxide, or fromthe hydrate produced by rehydrating magnesium oxide obtained bycalcining magnesium carbonate in granular form, for example. In anyevent, operation of the kiln is conveniently regulated by observing thetemperature at one selected point in the kiln, all other conditionsbeing kept constant. It has been found that when the temperature at anappropriate point in the kiln is kept constant, and all other conditionsare kept constant, a material ot a certain iodine number can beregularly obtained; vand that the iodine number of the product canL bevaried by varying the temperature at the specified point.

In general, it is desirable to operate in such a manner that the maximumtemperature to which the magnesla is subjected is relatively low, and sothat the time at maximum temperature is relatively short. Prolongedheating at high temperatures or localized overheating tend to produce amore dense product. And in general, anything which increases the densityof the product detracts from its adsorbent characteristics.

Fig. 3 shows one convenient type of apparatus for large scale operationsin accordance with this invention. An inclined rotary kiln I0, which intypical cases may be 170 feet long and 51/2 to 10 feet in diametenisturned by a motor III. A gas burner I2 controlled by a valve I3 directsa long flame (not shown) into the kiln. The iiameitselir may extend 10vor 20 feet into the kiln. Beyond such distance the iiame merges intoheated' exhaust gases. `An exhaust chest I4, advantageously providedwith a depending baffle I5 for dust separation, communicates with the'other end of the kiln and receives the exhaust gases therefrom,discharging them to a stack.

A filter cake or slurry of hydrated magnesia is f ed into the upper endof the vkiln through a hopperl or other suitable feeding means I6, andthe active, partially dehydrated product is discharged from the' lowerend of the kiln and withdrawn, as by `a conveyor II. A thermocouplepyrometer I8 is advantageously provided in the exhaust chest to indicatestack gas temperatures, and connected to a galvanometer I9. Thermocouplepyrometer 20, located as described below, is provided within the rotaryvkiln and is connected through collector rings 2 I, or in some othersuitable manner, with a galvanometer 22.

In one type of operation, a magnesium hydroxide slurry is fed` to arotary vacuum illter 23 and the filter cake, usually containing betweenand per cent solids (60 to 65 per cent water) isfed directly to the kilnas shown. The damp cake passes down the kiln as indicatedv at 24 and isbaked. Moisture is progressively driven off. 'I'he temperatureregistered by thermocouple 20 is observed, and after conditions havebeen stabilized, is kept constant. In a typical 170-foot kiln, theoptimum location of thermocouple 20 is about 40 feet from the dischargeend of the kiln. AThis location gives the most critical relation betweenthe characteristics of the product and the observed kiln temperature.The stack gas temperature is also watched (at I9). Should the' kilntemperature fall below the best value, the rotary speed of the kiln iscut down, or the'fuel feed is increased, or both. Converse steps aretaken if the kiln temperature rises above the desired value. For kilnsof the dimensions indicated, lthe temperaturc atpyrometer 20 will .oftenlie in the range from 600 to '700 C.

In one typical operationwith the apparatus described, the activatedmagnesia product issuing from the kiln has an iodine number of about andcontains about 10 parts H2O per 100 parts best to employ a smaller kilnfor this purpcse.-

For example, a rotary kiln 20 feet long and having an inside diameter ofabout 16 inches gives good results. This kiln is advantageously fed witha slurry of magnesium hydrate containing about 75 to 80 per cent ofwater. During operation of the kiln, a metallic cage f'is suspended froma chain about one-third of the way down from the feed end, in such amanner that the cage rolls with the descending charge and breaks up anylumps present or formed therein. The dehydration'of the magnesia in thiskiln is usually so regulated that the product contains from 6 to l5parts of water per 100 parts of MgO. More specically, the dehydration ofthe product is adjusted within the specified range in accordance with th'iodine number desired, and in accordance wit the further specificationthat the product is to be on the over-burned side, or the underburnedside, with respect to the iodine number peak. (See Fig. 1.)

As in the case of the larger kilns` previously described, these smallerkilns are readily controlled by maintaining a substantiallyxedtemperature at a certain point in the kiln. For example, it has beenfound that in one of these kilns,rma.intenance of a temperature of 285C. at a point 4 feet from the discharge end permits production of auniform product having an iodine number of about 90, when otherconditions have been properly adjusted, and are kept constant.

The location of the most critical point in the kiln for measuringtemperature (and consequently the best position for the thermocouple 20,Fig. 3) generally varies somewhat for kilns of different sizes andshapes. This is because rotary kilns and like. equipment varyconsiderably from one another, and because no single temperature ismaintained throughout a kiln during operation. Instead. a temperaturegradient exists, and it would be a vrare coincidence if two kilns shouldhave'the same temperature gradient. However, this is not a deterrent tothe practice of my invention, since the essential conditions in any kilnare substantially fixed, and the temperature gradient has a ratherdenite relationship to the temperature at a selected critical point.Furthermore, the location of the most critical point Iis readilydetermined. For example, a kiln is under these conditions is determined.The temperature is changed from time to time, and the point ofmeasurement ofthe temperature isalso variedto secure samples ofactivated magnesia produced at different temperature gradients, asindicated vby measurements at different points. Iodine numberdeterminations are then run on each sample to determine the temperaturegradient at which activated magnesia of maximum activity, or `otherdesired activity, is produced. When this data has been `obtained for anykiln, the kiln can readily be operated .under the conditions giving thedesired results by control of the temperature at a single point in thekiln. In other words, whenithe most desirable location for thethermocouple 20 has been established, the results from operations soconducted that a specified temperature is indicated by the thermocoupleare entirely predictable.

In this connection. it should be noted that it is notv always desirableto obtain a` product of maximum activity'. lFor example, in working oncarotene, it has been found that a" material having an iodine number offrom 50 to 70 is best. Such a material frequently contains nearly percent of residual MgiOH) 2, and about 20 per cent MgO,

and may therefore be considered as essentially an active magnesiumhydroxide. For other purposes, however, an adsorptive capacitycorresponding to an iodine number of 90 or 100 or 145 yor even more, maybe most suitable, and such l products usually contain much smalleramounts of residual Mg(OI-I) 2. The permissible water content in myactivated magnesia products can thus correspond to amounts of residualMg(OH) 2 from about 2 per cent to nearly 100 per cent.

' 'I'he baking or dehydrating operation in accordance with thisinvention involves countercurrent desiccation, in the sense that the wetmagnesia entering the kiln is subjected to' contact with'gases which arenearly saturated with water, while the more nearly ldehydrated magnesiaat the lower end cf the kiln is subjected to contact with hot flamegases which are substantially dry. This makes for a uniform dehydratingaction, and facilitates control, since the heat transfer is alsocountercurrent.

The magnesium hydroxide starting `material employed in the practice ofthis invention is important. In one of the best ways of preparingsuitable Mg(OH)z, a substantially sulfate-free magnesium salt solutionderived from a seawater bittern is treated with a CaO to precipitateMg(OH)2, and the precipitate is washed to re- .move chlorides,advantageously to less than 1 per cent, calculated as NaCl. Chlorides inexcess of this amount have a deleterious effect on the activity of thefinal product, and substantially reduce the maximum activity obtainable.In routine operation, it is best to precipitate Mg(OH)2 in the mannerdescribed, then wash it carefully, and feed the product to the kiln inthe form of a slurry or filter cake as described.

The activated magnesias made in accordance with this invention areuseful in various arts. I have employed them successfully inreconditioning used cleaning solvents, including Stoddards solvent, andCCl4; and also to remove sulfur compounds, including mercaptans, frompetroleum fractions. The utility of the activated magnesia product isnot limited to these applications. however.

What I claim is:

1. As a new article of manufacture, the decomposition product resultingfrom the heating of magnesium hydroxide, the product having activatedphysical and chemical properties and having 80 per cent to 85 per centof the original magnesium hydroxide content converted to magnesiumoxide.

2. In a method of manufacturing activated magnesium oxide, convertingprecipitated magnesium hydroxide to magnesium oxide under suchconditions as to effect conversion of the magnesium hydroxide betweenthe limits Aof about 80 per cent to 85 per cent.

3. The method of producing activated magnesium oxide comprising: heatingprecipitated magnesium hydroxide to a temperature in excess of 400 C.,maintaining a temperature in excess of 400 C., until only a substantialportion of the magnesium hydroxide is converted to magnesium oxide anduntil a minor but effective portion thereof remains unconverted, forsuch length of time as to produce a product having an absorptiveactivity, when compared to magnesium oxide obtained by calcination atdull red heat of precipitated magnesium hydroxide which has beendehydrated until the magnesium hydroxide content thereof is two percent. substantially in excess of said oxide. l

4. As a new article of manufacture, a decomposition product resultingfrom the heating of magnesium" hydroxide at a temperature of the orderofv400 C. for a limited time, the product having activated physical andchemical properties and consisting largely of magnesium oxide whilehaving a residual magnesium hydroxide content lying above 2 per cent.

5. The method of manufacturing activated magnesia which comprisesheating magnesium hydroxide to convert it largely to magnesium oxide,and controlling the heating to effect conversion of the magnesiumhydroxide to such extent as to yield a product having a residual contentof unconverted magnesium hydroxide lying above 2 per cent.

6.. 'Ihe method of manufacturing activated magnesia which comprisesheating precipitated magnesium hydroxide to convert it largely tomagnesium oxide, and controlling the heating to effect conversion of themagnesium hydroxide to such extent as to yield a product having aresidual content of unconverted magnesium hydroxide lying above 2 percent.

7. As a new article of manufacture, a decomposition product resultingfrom the heating of magnesium hydroxide at a temperature of at least 400C. for a limited time, the product having an iodin absorption number inexcess of 60.

8. The method of manufacturing activated magnesium oxide which comprisesconverting magnesium hydroxide to a highly activated product by heatingsaid magnesium hydroxide to a temperature of at least 400 C. for asufcient time to raise the iodin absorption number of the product to avalue in excess of 60.

.9. The method of manufacturing activated magnesia which comprisesconverting precipitated magnesium hydroxide to a highly activatedproduct by heating said magnesium hydroxide to a'. temperature of atleast 400 C. for a suillcient time to raise the iodin absorption numberof the product to a value in excess of 60.

10. An adsorbent material, for use as in decolorizing oils, containingabout 90 per cent of MgO and the balance substantially entirely Mg(OH) nand having an adsorbent value of about 100 on the iodin scale.

11. A process for producing a magnesium material, said processcomprising calcining a substantially pure Mg(OH)z for about 15 minutesat a temperature of approximately 660 C. to produce a material useful asan adsorbent. e. g. in de'colorizing oils.

12. In the production of activated magnesia of high adsorbent power, theprocess which comprises briefly heating magnesium hydroxide to atemperature above 350 C., the time of heating and the temperature beingso correlated in inof magnesium oxide slurry through a rotary kiln,.

the speed of feed and heating being so correlated that the `emergingmagnesia has an iodine value hydroxide at a temperature above 350 C. fora time sumcient to give an iodin value of 60.

MAX Y. SEATON.

